The oldest and still most economical method for making crude phosphoric acid is to treat phosphate rock with a mineral acid, i.e. sulfuric, phosphoric or nitric, thereby precipitating calcium salts and releasing phosphoric acid. Phosphoric acid so produced is used, in turn, both in treating phosphate rock for production of triple superphosphate and in reaction with ammonia to make ammonium phosphates. The growing importance of these products in the fertilizer industry makes phosphoric acid a very important intermediate. To a lesser extent, phosphoric acid can be used directly in some fertilizer applications and as a reagent in certain industrial processes.
Phosphoric acid produced by treatment of the phosphate rock with sulfuric acid is commonly known as "wet-process" or "green" acid. Production of phosphoric acid by the wet process generally involves dissolving the phosphate rock in sulfuric acid, holding the acidulate slurry until the calcium sulfate crystals grow to sufficient size, separating the acid and calcium sulfate by filtration and concentrating the acid to the desired level.
The separtation of the phosphoric acid from the calcium sulfate by filtration represents one of the main possibilities for further refinement of the wet process. Prefilt wet process acid is very impure in that it contains a wide variety of materials. These materials include organic material, silica and compounds of iron, aluminum, calcium and fluorine. Filtration of this acid slurry is an extremely difficult operation which requires foremost among other factors crystals of sufficient size, shape and strength for filtration to be effective. Very fine crystals have been found to result in filter blinding and crystals of insufficient strength will break apart under the filtration forces. Continuous production of satisfactory crystals has been dependent upon the proper sulfate concentration, slurry recirculation, phosphoric acid concentration and impurities in the processed rock. Although refinements in the filtration process have been the object of considerable development work, there yet exists the need for further improvements in this difficult operation.